Casillas, 37, would not confirm the decision, saying in a tweet that "a day will come when I will have to retire. Leave me to announce that news when the time comes."
Porto president Jorge Pinto da Costa Pinto told O Jogo that the Portuguese club was already looking for a new goalkeeper to replace Casillas, but he wanted to keep the Spaniard at the club with a yet unspecified role.
Porto did not respond to a request for comment.
Casillas added that he had a check up with his doctor on Thursday and that "all was well."
Casillas, who joined Porto in 2015, played 725 times for Real Madrid, winning three Champions Leagues and five La Liga titles. With Spain, he won the World Cup in 2010.
Casillas was released from hospital on May 6 after suffering a heart attack in training.
(Reporting By Andrei Khalip; Editing by Andrew Cawthorne and Keith Weor)
